The overall objective is to assess the usefulness of rabbit anti-mouse brain serum as a tool in characterizing murine hematopoietic stem cells. Specific goals include: an assessment of the specificity of this antiserum for pluripotent as compared to unipotent stem cells; an evaluation of the crossreactivity of the serum with thymus-derived cells and 'helper' cells involved with spleen colony formation by stem cells; an evaluation of the properties of stem cells resistant to the antiserum as an approach to defining immunologically distinct stem cell subpopulations; and an attempt to identify pluripotent stem cells directly with the antiserum by immunofluorescence and cell sorting. The availability of an immunological tool to dissect the events which occur in early hematopoiesis should provide the means for a better understanding of how hemopoiesis is regulated at the stem cell level as well as on the unique characteristics of cells participating in these early events.